1. Field of the Invention
The present invention relates to a process of drawing an optical fiber. In particular, the present invention relates to a process of drawing an optical glass fiber having increased reliability and reduced transmission loss by preventing formation of defects and decreasing thermal fluctuations in a molecular order or an atomic order in the glass fiber.
2. Description of the Related Art
When an optical glass fiber is conventionally drawn with melting a glass preform for the optical fiber, the preform is heated to a temperature above its softening point in a drawing furnace (or a melt drawing furnace) and the optical fiber is drawn from the preform followed by coating the fiber with a resin composition with a die after allowing the fiber to be air-cooled.
As shown concretely in FIG. 1 which schematically illustrates a drawing apparatus used in a conventional process of drawing the optical fiber, a muffle tube 102 into which a preform 101 for the optical fiber is inserted is supported by a furnace body 104 together with a carbon heater 103 surrounding the tube. To an upper end of the furnace body 104, is connected a purging gas tube 106 which supplies an inert gas such as nitrogen in the drawing furnace 105 in order to prevent degradation of the muffle tube 102 and the carbon heater 103 due to oxidation. A sealing ring 107 is fixed around an upper end surface of the furnace body 104 to be in sliding contact with an outer surface of the preform 101, and a shutter 108 is provided at a lower end of the furnace body 104, whereby an inert atmosphere can be kept in the drawing furnace 105. The optical fiber 109 drawn in the furnace 105 is passed through a coating die 111 and applied with a first coating of, for example, an ultraviolet curing resin. The ultraviolet curing resin which passes through the coating die 111 together with the optical fiber is cured by irradiation of ultraviolet in an ultraviolet curing device 113 so as to integrate the resin with the optical fiber 109. At a lower end of the device 113, a tube 114 is provided which supplies a purging gas such as nitrogen to exclude oxygen which adversely affects the curing of the resin. In connection with the gas tube, a shutter 115 is provided at an upper end of the device 115 in order to prevent outflow of the purging gas from the device 113. In addition, along the downstream from the curing device 113, another coating die 116 for a second coating and another curing device 117 are provided in turn, so that the optical fiber 118 coated with the first coating is converted to an optical fiber 119 which is further coated with the second coating. The optical fiber 119 is wound up by a winder (not shown) via a direction changing roller 120.
In the conventional process, since the optical fiber is drawn directly into an atmosphere at an ambient temperature from the drawing furnace, defects are fixed and remain in the fiber due to quenching of the optical fiber. In addition, since the glass preform is kept in the furnace at a high temperature (higher than 2000.degree. C.), thermal fluctuations in the glass are promoted. Solutions to overcome these problems are highly desired in order to reduce a transmission loss of the optical fiber and to make the reliability of the optical fiber higher by means of preventing the increase of the transmission loss over a long period.